The present invention relates to a method and system for adsorbing carbon dioxide (CO2) from a breathable gas stream which gas stream is derived from a dosed habitable environment and which method and system are operative to regenerate an adsorbent on or in which the CO2 is adsorbed. More particularly, the method and system of this invention utilize a sweep gas stream having a low CO2 partial pressure to strip adsorbed CO2 from the adsorbent. The low CO2 partial pressure sweep gas stream is produced by expanding the volume of a portion of the breathable gas stream after the latter has passed through the CO2 adsorption station in the system. The remainder of the CO2 desorbed breathable gas stream is returned to the habitable environment. An expanded portion of the breathing gas could also be used as a desorption sweep gas for the adsorption bed. This option would require a slightly higher volume of the sweep gas flow, but would allow decreasing the size and pressure drop of the CO2 removal system. The choice of the aforesaid options will depend on system level factors which include the gas circulation assembly and the system operating pressure.
Carbon dioxide (CO2) removal systems which continuously or cyclically dump the removed CO2 (and possibly water vapor) to an ambient environment have seen substantial development in recent years. They offer significant reductions in the on-back weight of regenerable EVA life support systems and are therefore highly desirable for future systems. One major problem with their use has been the fact that they will not work in environments, for example, like Mars, where the ambient atmosphere contains CO2 at a partial pressure near or above the acceptable values in the space suit breathing atmosphere. Under these conditions, desorption or transport processes which are partial pressure driven are incapable of stripping adsorbed metabolic CO2 from an adsorption station so as to enable the adsorption station to produce a breathable atmosphere.
Solutions to the problem which have been proposed include the addition of thermal energy to allow desorption at higher partial pressures and the use of regenerative combinations of compressors and turbines to achieve a higher gas pressure from which the transport gradient is favorable. Both of these approaches require the addition of appreciable energy during operation and of added equipment to the system. This adds weight and system complexity making the system less desirable for long planetary exploration missions.
It would be desirable to provide a regenerable CO2 adsorption system which is compact, light in weight, and can operate satisfactorily in an ambient atmosphere such as that which exists on the planet Mars.
This invention relates to a method and system of dumping stripped CO2 into an ambient environment which ambient environment having an atmosphere which has a relatively high CO2 partial pressure, such as the atmosphere on the planet Mars. The system and method of this invention are extremely uncomplicated and require minimal energy input. The system and method of this invention make use of a small divergent flow of a CO2-cleansed air flow stream, which cleansed air flow stream is derived from the space suit or space station ventilation loop downstream of the CO2 adsorption station. In an ejector-driven system of the type described hereinafter, the diverting of a portion of the CO2-cleansed air stream from the ventilation loop will create no significant operational or design problems. A bleed valve is all that is necessary in the ventilation loop to divert the portion of the CO2-cleansed air stream to the CO2 desorption station of the system. The bleed valve can be periodically opened by a habitat controller, such as a microprocessor computer, or the like, if so desired.
The system and method of this invention make use of an ambient atmosphere pressure that is well below the pressure which is maintained in the habitable environment The system and method of this invention utilize a small diverted fraction of the recirculating habitat ventilation flow stream which diverted fraction is expanded in a CO2 desorption station, the latter of which is maintained at a pressure which is near ambient pressure so as to allow gas flow through the CO2 desorption station and thence to the ambient environment This pressure drop produces a proportionally reduced partial pressure of CO2 in the sweep gas stream flowing through the CO2 desorption station. When the volumetrically expanded sweep gas stream flows past a membrane exposed to the CO2-laden vent gas in the space suit, or through a loaded sorbent bed, the favorable CO2 partial pressure gradient which is created will result in the removal of CO2 through the membrane, or from the sorbent bed, to the expanded sweep gas stream. Afterwards, the CO2 laden sweep gas stream is discharged into the surrounding ambient environment The amount of CO2 which can be effectively removed is dependent on the volumetric flow of the sweep gas steam and it can be adjusted to equal the amount of CO2 which is adsorbed from the habitat vent gas stream. The system is self balancing over a broad range of operating conditions since the sorbent or membrane will allow the CO2 partial pressure to rise or fall until the amount of CO2 removed in desorption is equal to the amount of CO2 adsorbed at any given flow rate. A varying partial pressure in the vent loop which exists over a moderate range is typically acceptable, unless the partial pressure value exceeds upper limits for a breathable atmosphere. The amount of CO2 which is desorbed from the sorbent can be adjusted to equal the amount of CO2 which is adsorbed from the habitat vent gas stream. Thus the adsorption desorption values can be put into equilibrium with each other.
In a Mars planet space-suit scenario, the pressure inside the space suit is likely to be 3.8 psia (approximately 200 mm Hg), and the Martian ambient pressure is in the range of approximately 6 to 8 mm Hg. Expansion of the diverted sweep gas creates a sweep gas pressure which is about 16 mm Hg which will permit a sonic discharge to the ambient atmosphere thus ensuring good isolation, i.e., no back flow, by providing a 12:1 volumetric expansion of the diverted sweep gas stream. Only about 8% of the recirculating gas flow stream would be required to be diverted in order to ensure a satisfactory level of CO2 desorption from the CO2 adsorption station. There is a need to keep the space suit volume and the desorption or membrane diffusion parts of the carbon dioxide removal system at a substantially lower carbon dioxide partial pressure than the surrounding atmosphere into which the system ultimately discharges. Thus back flow and back diffusion must be suppressed. High velocity organized flow through a limited discharge area will accomplish this object
It is therefore an object of this invention to provide a method and system for removing CO2 from a recirculating gas stream by means of a CO2 adsorbent station, which gas stream is derived from a breathable atmosphere in a dosed habitable environment such as a space suit or a space station.
It is an additional object of this invention to provide a method and system of the character described wherein the CO2 is stripped from the CO2 adsorbent station by means of a diverted volumetrically expanded fraction of the CO2-cleansed recirculating gas stream.
It is a further object of this invention to provide a method and system of the character described wherein the partial pressure of CO2 in the volumetrically expanded fraction of the recirculating gas stream is lower than the partial pressure of CO2 in the surrounding ambient